1. Field of the Invention
The present invention relates to ophthalmic lenses. More particularly, the present invention is directed to ophthalmic lenses having power profiles that provide foveal vision correction and minimize visual acuity variation experienced by progressing myopes over the course of time.
2. Discussion of the Related Art
Common conditions which lead to reduced visual acuity include myopia and hyperopia, for which corrective lenses in the form of spectacles, or rigid or soft contact lenses, are prescribed. The conditions are generally described as the imbalance between the length of the eye and the focus of the optical elements of the eye, myopic eyes focusing in front of the retinal plane and hyperopic eyes focusing behind the retinal plane. Myopia typically develops because the axial length of the eye grows to be longer than the focal length of the optical components of the eye, that is, the eye grows too long. Hyperopia typically develops because the axial length of the eye is too short compared with the focal length of the optical components of the eye, that is, the eye does not grow long enough.
Myopia has a high prevalence rate in many regions of the world. Of greatest concern with this condition is its possible progression to high myopia, for example greater than five (5) or six (6) diopters, which dramatically affects one's ability to function without optical aids. High myopia is also associated with an increased risk of retinal disease, cataracts, and glaucoma.
Corrective lenses are used to alter the gross focus of the eye to render a clearer image at the retinal plane, by shifting the focus from in front of the plane to correct myopia, or from behind the plane to correct hyperopia, respectively. However, the corrective approach to the conditions does not address the cause of the condition, but is merely prosthetic or intended to address symptoms. Moreover, conventional corrective approaches for spherical equivalent power, such as spectacles and contact lenses, are effective at correcting the static focus error, but do not address the dynamic change in gross focus error over time, resulting in poor vision outcome shortly after the dispensing of lenses.
Most eyes do not have simple myopia or hyperopia, but have myopic astigmatism or hyperopic astigmatism. Astigmatic errors of focus cause the image of a point source of light to form as two mutually perpendicular lines at different focal distances. In the following discussion, the terms myopia and hyperopia are used to include simple myopia or myopic astigmatism and hyperopia and hyperopic astigmatism respectively.
Over the course of a year, young myopes usually progress toward more severe myopia. The progression rate typically ranges from about −0.25 D/year to about −0.75 D/year. If a conventional optical correction (e.g., with spherical spectacle glasses) is prescribed for a progressing myope to provide optimal distance vision correction at the beginning of the year, then at the end of the year, the patient may experience mild-to-severe blurry distance vision. The present invention seeks to provide lens designs that minimize such a variation in vision and prolong the period before the subject feels the need for a new prescription.